Project Summary Autism spectrum disorder (ASD) currently affects 1 in 59 children in the United States. Prenatal exposure to environmental factors like air pollution, which activate the immune system, have been associated with increased ASD risk. However, the mechanisms by which adverse environmental exposures during pregnancy lead to altered maturation of the fetal brain remain unknown. Microglia, the resident immune cells of the brain, are key regulators of both the neural response to immune activation and the developmental organization of neural circuits, making them uniquely poised to translate such adverse environmental exposures into neural outcomes. Interestingly, recent studies suggest that prenatal challenges can alter the trajectory of brain development (maturation), leading to aberrant neural circuit formation. We recently developed the microglial developmental index (MDI) to objectively measure the global maturational state of microglia based on transcriptomic sequencing. In mice, we showed that the MDI is accelerated by an acute immune challenge in males only. Using human datasets, we found that the MDI is higher in ASD patients than in controls. Together, these findings suggest that changes in microglial maturation represent a potential mechanism by which immune insults increase ASD risk in sex-specific ways. Our lab has developed a novel mouse model of prenatal immune activation which combines exposure to diesel exhaust particles (DEP) with a maternal stressor (resource deprivation; MS). My preliminary data demonstrate that DEP/MS exposure impairs social behavior in male offspring only. Therefore, in Aim 1, I will use next-generation RNA sequencing on isolated microglia to test the hypothesis that DEP/MS exposure will accelerate the MDI in males only. My preliminary data also show that DEP/MS exposure decreases dopamine D1 receptor (D1R) mRNA in the nucleus accumbens (NAc). Moreover, we recently found that microglia-mediated synaptic pruning is critical to the natural development of D1R in the NAc, and social behavior. Thus, in Aim 2, I will test the hypothesis that DEP/MS exposure increases microglial pruning of NAc-D1Rs in males only. Finally, in Aim 3 I will test the hypothesis that DEP/MS-induced changes in NAc-D1R are causal to DEP/MS-induced deficits in social behavior. Specifically, I predict that a) local inhibition of microglial pruning in the NAc will prevent social behavior deficits following DEP/MS and b), if these effects are dependent on D1R signaling specifically, then this restoration will be attenuated by concurrent NAc-D1R antagonism. Together, this work will have an important positive impact on both our basic understanding of the developmental biology of microglia, as well as the specific contribution of neuro-immune signaling to the etiology of ASD.